*3.4. Conductivity of the Ion-Modified Layer*

It is known that recrystallization during diamond annealing after ion implantation at temperatures <60 ◦C occurs only at low irradiation fluences, which do not lead to a vacancy concentration of more than 10<sup>22</sup> cm−<sup>3</sup> [25]. The isochronous annealing of the implanted diamond with a final temperature *T*<sup>a</sup> = 1200 ◦C leads to an almost initial structure. At high irradiation fluences, a disordered irradiated diamond layer undergoes an irreversible transition to graphite-like structures under annealing. The processes of the graphitization of thin layers in diamond under ion irradiation are of interest for the creation of conductors on the surface of diamond and various diamond–graphite heterostructures [26–28].

Due to the dielectric properties of diamond, the analysis of the conductivity of modified layers is a simple and effective method for assessing ion-induced structural changes. This is demonstrated by the dependences of the conductivity σ = (*R*s·*t*) <sup>−</sup><sup>1</sup> of the modified diamond layers on the irradiation and annealing temperatures shown in Figure 5. The dotted lines in the figure show the range of conductivity values typical for prepregs of synthetic graphites in the process of graphitization at *T*<sup>a</sup> > 1000 ◦C. It can be seen that the conductivity of the irradiated-at-30 ◦C diamond layer, comparable to that of graphitized carbon materials, begins at *T*<sup>a</sup> > 300 ◦C. The increasing character of the σ (*T*a) dependences suggests that an increase in *T*<sup>a</sup> > 700 ◦C will lead to an even greater increase in the conductivity of the irradiated layer. It should be noted that at *T*<sup>a</sup> ≥ 1300 ◦C, the transformation of diamond into graphite begins. More efficient graphitization occurs with "hot" irradiation (Figure 5) (see [11]). A sharp increase in conductivity begins at a *T* of about 150 ◦C; then, after passing the maximum of about 300 ◦C, a decrease in the conductivity is observed, which is small for irradiation with argon and significant for irradiation with carbon ions.

**Figure 5.** Dependences of the conductivity σ of the irradiated polycrystalline diamond layers on the irradiation temperature *T* and annealing *T*a.

A decrease in conductivity at 400–600 ◦C is associated with the formation of nanocrystalline graphite, which has a lower conductivity [11,12]. It can be seen from the figure that a graphite-like layer can also be obtained by irradiating diamond with carbon ions at a temperature corresponding to the maximum conductivity of the modified layer. Thus, it can be assumed that irradiation with carbon ions makes it possible to process the diamond either with the formation of a graphite layer or with the growth of the diamond surface. Vacuum arc sources, which have successfully been used for both coating deposition [5–9] and for implantation [29], can be suitable generators of carbon ions.

#### **4. Conclusions**

The graphitization and growth of diamond surfaces by the high-fluence (>10<sup>18</sup> ion/cm2) irradiation with 30 keV argon and carbon ions of CVD diamond and the (111) face of HPHT diamond were experimentally studied.

To characterize the high-fluence ion irradiation, the depth distributions ν(*x*) of the numbers of displacements per atom (dpa) were calculated using the SRIM code. Based on the calculated profiles of ν(*x*), the thickness of the modified surface layer was estimated for Ar<sup>+</sup> and C<sup>+</sup> ion irradiation. Unlike for irradiation for argon ions, for carbon ion irradiation, the thickness of the modified layer increased due to the implanted carbon ions.

SEM and AFM showed the removal of traces of mechanical polishing under Ar<sup>+</sup> and C<sup>+</sup> ion irradiation and the appearance of a stochastic microrelief. The roughnesses before and after the ion irradiation of the diamond surface are close—the root-mean-square roughness at a base length of 1 μm is about 20 nm.

Raman spectroscopy before and after the CVD diamond irradiation showed the graphitization of the surface layer when irradiated with argon ions at the temperature of 230 ◦C and a diamond structure for the synthesized layer when irradiated with carbon ions at the temperature of 650 ◦C.

The measurement of the conductivity of the modified layer on diamond is a simple and effective method for determining the irradiation or annealing temperature to obtain a graphitized modified layer. Graphite conductivity is obtained at a diamond irradiation temperature of about 300 ◦C. Annealing a diamond irradiated at room temperature can also lead to a graphitized layer. The measurement of the electrical conductivity of irradiated CVD diamonds showed that the graphitization of the surface layer can also be obtained by irradiation with carbon ions.

**Author Contributions:** Conceptualization, S.N.G.; methodology, I.V.S.; validation, E.S.M.; investigation, A.M.B., V.A.K., M.A.O. and E.S.M.; writing—original draft preparation, A.M.B.; writing—review and editing, E.S.M. and M.A.O.; project administration, S.N.G. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by the Ministry of Science and Higher Education of the Russian Federation, grant number 0707-2020-0025.

**Conflicts of Interest:** The authors declare no conflict of interest.
